Biofuels are of increasing interest for a number of reasons including: (1) they are a renewable resource, (2) their production is less dependent on geopolitical considerations, (3) they provide the possibility of a direct replacement of petroleum-based fuels in existing vehicles, and (4) the net greenhouse gas emissions can be substantially reduced by virtue of CO2 uptake by biofuel precursors—particularly in the case of cellulosic feedstocks. See Pearce, “Fuels Gold,” New Scientist, 23 September, pp. 36-41, 2006.
An easily-obtainable biofuel is vegetable oil, which largely comprises triglycerides and some free fatty acids. The properties of vegetable oil, however, make it generally inappropriate for use as a direct replacement for petroleum diesel in vehicle engines, as the vegetable oils' viscosities are generally too high and do not burn cleanly enough, thereby leaving damaging carbon deposits on the engine. Additionally, vegetable oils tend to gel at lower temperatures, thereby hindering their use in colder climates. These problems are mitigated when the vegetable oils are blended with petroleum fuels, but still remain an impediment for long-term use in diesel engines. See Pearce, 2006; Huber et al., “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering,” Chem. Rev., vol. 106, pp. 4044-4098, 2006.
Transesterification is currently a method used to convert vegetable oils into diesel-compatible fuels that can be burned in conventional diesel engines. See, e.g., Meher et al., “Technical Aspects of Biodiesel Production by Transesterification—A Review,” Renew. & Sustain. Energy Rev., vol. 10, pp. 248-268, 2006. However, a similar cold flow problem with conventional biodiesel fuels still remains. This problem is at least partly due to the fact that at lower temperatures, e.g., around freezing (ca. 0° C.), biodiesel often thickens and does not flow as readily. Conventional biodiesel is primarily composed of methyl esters which have long straight chain aliphatic groups attached to a carbonyl group. Also, the transesterification of vegetable oils exhibits a problem of producing more than 90% diesel range fuels with little or no kerosene or gasoline range fractions, thereby limiting the types of fuels produced therefrom. For the conversion of vegetable and other oils to some fuels (e.g., non-diesel), it is likely that the oils must first be converted to alkanes (paraffins).
It is also worth noting that unsaturation in the fatty acids (obtained from the vegetable oil) contributes to poor oxidation stability and deposits, and that while hydrogenation will generally improve the oxidation stability of the fuel, it can make the already poor low temperature performance of the fuel even worse. Isomerization of the paraffins can ameliorate this problem.
Accordingly, methods and systems for efficiently processing vegetable and/or crop oils into a broader range of fuel types and lubricants, often simultaneously, would be highly beneficial.